The invention relates generally to a method in paper web reeling apparatus wherein the web is introduced over the mantle of a reeling cylinder, which is preferably driven, and is then passed onto a reeling shaft on which the web is reeled into a roll.
The invention further relates to a paper web reeling apparatus comprising a driven reeling cylinder over which a web is introduced and then passed onto a reeling shaft on which the web is reeled.
One type of web reeling apparatus to which the invention is applicable is the so-called Pope-type reeling apparatus commonly used for reeling paper web as the web leaves, for example, a paper making machine, a coating machine, a supercalender or printing machine. The web is reeled onto a shaft to form a roll which is pressed against the Pope or reeling cylinder over a sector of which the web runs and which is rotatably driven so that its circumfenterial speed corresponds to the speed of the incoming web. Before the roll is completed, a new reeling shaft is brought into nip contact with the Pope cylinder so that the new reeling shaft begins to rotate at a corresponding circumferential speed. As soon as the paper roll is completed, i.e., obtains the desired diameter, it is shifted away from the Pope cylinder where upon its rotational speed begins to decrease resulting in the formation of a web loop between the new reeling shaft and the completed roll. The web loop is guided, for example, by means of a jet of compressed air, to wind around the new reeling shaft whereupon the web is torn from the completed roll.
A problem has been encountered in the reeling of certain paper qualities, for example, LWC and SC. In particular, in the case where a web of such paper qualities is introduced into the reeling apparatus at higher speeds, the web to be reeled tends to slide on the surface of the reeling cylinder. Such sliding particularly occurs in reeling apparatus of the type described above, i.e., wherein the new paper roll being formed on the reeling shaft is rotated through nip contact of the roll with the circumference of the driven reeling cylinder utilizing the friction force that exists between the paper web and the surface of the reeling cylinder. Thus, if the friction force is below a certain minimum value, the incoming paper web will slide on the surface of the reeling cylinder which results in uncontrolled variations in the web tension and hardness in the paper roll being formed. These variations in tightness and hardness produce wrinkles in the rolls and particularly at the inner layers of the rolls so that the innermost portions of the rolls are useless. This in turn results in considerable economic losses for the paper mill.
The extent to which the paper web slides over the surface of the reeling cylinder depends on several factors, such as the tightness with which the web is held against the surface of the reeling cylinder and the linear load produced by the primary and secondary forks which hold the paper roll being formed against the reeling cylinder. Moreover, the sliding is affected by the surface properties of the paper being reeled and of the reeling cylinder, i.e., by the coefficient of friction between the surfaces. The coefficient of friction is to a great extent determined by the introduction of air between the web and the surface of the reeling cylinder which tends to separate the web from the reeling cylinder.
The problem of sliding of the paper web on the surface of the reeling cylinder has in the past been treated by setting the tension of the paper web being introduced in the paper apparatus to be as high as possible through the adjustment of a speed differential between the reeling cylinder and the immediately preceeding drive unit from which the paper web leaves for introduction into the reeling apparatus. However, the amount to which the tension of the web can be increased is limited by the tendency of the web to break and by deterioration of the quality characteristics of the paper since, for example, the tensile strength and the "stretch at break" in the region of the break are thereby reduced.
Another technique used in attempting to prevent sliding between the web and the reeling cylinder surface is through increasing the linear loads between the roll being formed and the reeling cylinder to the maximum extent possible by using an excessively high loading force for the secondary or supporting forks by which the roll is pressed against the reeling cylinder. One drawback of this technique is that the quality characteristics of the paper deteriorate due to reductions in the tensile and the "stretch at break" of the web.
With respect to prior art related to the present invention, reference is made to Finnish patent application No. 84 3747 of Oy Kaukas Ab and Valmet Oy. A Pope-type reeling apparatus is disclosed wherein the paper roll being wound on a roll shaft is rotated through nip contact with the cirumference of the mantle of a reeling cylinder. In order to prevent sliding between the paper web and the mantle of the reeling cylinder, the interior of the reeling cylinder is maintained at a negative pressure within the sector of contact between the paper web and the reeling cylinder. By means of the negative pressure, the friction between the paper web and the surface of the mantle of the reeling cylinder is substantially increased since the pressure differential between the ambient atmosphere and the interior of the reeling cylinder causes the paper web to be held tightly against the surface of the reeling cylinder and, at the same time, removes any air which is wedged between the paper web and the cylinder surface.
During operation of conventional web reeling apparatus, air flow is induced into the wedge-shaped space or inlet nip defined between the incoming run of the web and the reeling cylinder mantle which creates a detrimental positive pressure in the inlet nip. This positive pressure is a result of the boundary-layer flows which are carried with the moving web and cylinder mantle. Due to the positive pressure nip, air flows between the web and the cylinder surface which reduces the friction between the web and cylinder. Moreover, air bubbles are formed which cause wrinkles in the web or which can even break the web.
The problems discussed above have increased as the speeds of paper machines have increased. For this reason, on-machine reeling apparatus have become one of the limiting factors preventing further increases in paper machine speeds. Moreover, these problems are particularly acute in the case of reeling coated papers which are slippery and impervious.
Another prior technique which has been used in an attempt to overcome the web sliding problem is the use of a grooved mantle for the reeling cylinder. It has been suggested that the air flow induced into the inlet nip can escape through the grooves so that no air bubbles are produced between the web and the cylinder surface. However, in order that the grooves will not cause marking of the web as it is reeled, it has been necessary to form the grooves with very small widths which, consequently, results in the grooves becoming blocked with paper fiber and dust. Accordingly, after a relatively short time, the grooves fail to operate as intended. To clean the grooves is a time consuming and difficult operation. For this reason, the provision of grooves on the reeling cylinder has not overcome the web sliding problem. Although the Pope cylinder construction described above in connection with Finnish application No. 84 3747 efficiently prevents the formation of an air layer between the web and the cylinder surface, the construction is quite expensive in manufacture.